Among the known methods for forming laminations in micro-fabricated high frequency inductors and transformers, the utilization of thick, non-removable photoresist (PR) is the most efficient since it provides a high volume of magnetic material while maintaining the small thickness of the film, which is important for minimizing eddy current losses.
FIG. 1 shows a patterned layer of non-removable negative photoresist 100 formed on a substrate 102, e.g., a semiconductor substrate such as crystalline silicon. The patterned photoresist layer 100 includes a number of vias 104 formed to expose upper surface regions of the substrate 102. As shown in FIG. 1, each via 104 is lined on its bottom surface and sidewalls with copper seed material 106. A magnetic alloy lamination 108 is formed on the copper seed material 106.
With the continuous increase in frequency of operation in many IC applications (e.g., switching frequency of a buck converter), skin effect contribution into the power loss becomes more pronounced. An obvious solution is plating thinner magnetic films. However, this has the undesirable effect of reducing the cross-sectional area of the magnetic core which, in turn, linearly reduces the inductance.